1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to an Over Temperature Delta Temperature and Over Power Delta Temperature operating margin recovery method for increasing the Over Temperature Delta Temperature and Over Power Delta Temperature setpoints of a nuclear Reactor Trip System thereby increasing the safe operating margin of the reactor. The invention further relates to a reactor system, such as the Reactor Trip System, employing an Over Temperature Delta Temperature, Over Power Delta Temperature recovery method.
2. Background Information
To ensure that the specified acceptable fuel design limits of a nuclear reactor, such as a pressurized water reactor (PWR), are not exceeded, a Reactor Trip System (RTS) is typically employed. The RTS is designed to automatically initiate the rapid insertion of the control rods, commonly referred to as the reactor scram function, of the reactivity control system by interrupting electrical power to the rod control system and allowing the control rods to fall by gravity into the reactor core. Generally, the RTS includes a variety of different devices (e.g., without limitation, power sources, sensors, communication links, software/firmware, initiation circuits, logic matrices, bypasses, interlocks, switchgear, actuation logic, and actuated devices) which are required to initiate a reactor trip. Using such devices, the RTS initiates a trip and shuts down the reactor when established setpoints are surpassed.
Among the reactor trip functions the RTS provides are, for example, core heat removal trips including, without limitation, an Over Power Delta Temperature (OPDT) trip, which is designed to effectuate a reactor trip in order to protect against excessive power (i.e., fuel rod rating protection), and an Over Temperature Delta Temperature (OTDT) trip. Conventionally, the OTDT and OPDT setpoints for such trips are calculated based upon dynamic compensations of measured temperature differences in both the hot leg and cold leg of the Reactor Coolant System (RCS), the average of hot leg and cold leg temperature, and the core power distribution in the core of the reactor. However, due to the method of measuring temperature in the hot leg and due to the temperature streaming in the hot leg, steady state temperature fluctuations exist which adversely challenge the OPDT and OTDT trip setpoints.
More specifically, several nuclear power plants have been known to experience aperiodic hot leg temperature (Thot) fluctuations that originate, for example, in the reactor vessel upper plenum and wherein the temperature in the hot leg rapidly increases by about 1° F. to about 3° F. in a fast ramp up, remains at an elevated temperature for several seconds, and finally returns to the original temperature. Among other disadvantages, such temperature oscillations undesirably lead to a reduction in the OPDT or OTDT safe operating margin. In turn, this could result, for example, in a partial turbine runback, an actual turbine runback (i.e., when more than one channel is affected) or, in the extreme case, a reactor trip. Such hot leg temperature fluctuations are attributed to the aforementioned hot leg flow streaming phenomenon of the Reactor Coolant System (RCS). Such flow streaming adversely impacts the measured RCS average temperature, which is often used as an input to the Rod Control System and, therefore, can cause spurious control rod stepping when the Rod Control System is in automatic mode. In order to avoid such rod stepping, several power plants have been forced to operate in manual rod control mode. Additionally, in an attempt to accommodate the aforementioned fluctuations, a plurality of filters have been required to be employed in the RTS in order to provide filtering functions in both the cold and hot legs, and after the average temperature has been obtained for both legs.
There is a need, therefore, for a modification of protection functions of the RTS and, in particular, of the Over Power Delta Temperature (OPDT) and Over Temperature Delta Temperature (OTDT) trip functions.
There is, therefore, room for improvement in OPDT and OTDT operating margin recovery methods and in reactor systems employing the same.